1. Field of the Invention
The present invention relates to a direct current (DC) charging step using a charge roller in an image forming apparatus such as copying machine, printer, facsimile machine, or multifunction product.
2. Description of the Related Art
In an image forming apparatus that employs an electrophotographic process, an image is formed by charging a photoconductor, and conducting light exposure, development, transfer, and fixing. In a charging step (i.e., for charging the photoconductor), as a charger a scorotron charger has been conventionally used, and as a charger roller a charge roller that generates lesser harmful gas, such as ozone, NOx, from the viewpoint of bad effect on environment and realizes downsizing of apparatus is used.
In the charging step, a so-called alternate current (AC) charging system which applies voltage in which AC voltage is overlapped with DC voltage has been used (for example, see Japanese Patent Application Laid-open No. 2001-194868 and Japanese Patent Application Laid-open No. 2005-309073). The AC charging system can charge a photoconductor effectively, so that charge potential of the photoconductor is easy to be uniformized. In the AC charging system, however, positive charging and negative charging occur a number of times within a period of a second, corresponding to the frequency of AC voltage. Because the energy that is generated in a single charging is large enough to decompose an organic substance by oxidization, the photoconductor and the charge roller are oxidized and deteriorated early. Furthermore, because the surface of photoconductor and surface of charge roller which are oxidized by AC charging are more susceptive to adhesion of toner ingredients (toner resin, wax, externally added substances (such as silica or titanium oxide)) and ingredients of paper, these substances having adhered to the photoconductor or the charge roller may not be removed. When such a phenomenon occurs in a photoconductor, surface resistance of the photoconductor at high humidity environment decreases, and a latent image is equalized, leading the phenomenon of image bleeding. Further, in a charge roller, resistance of the charge roller partially elevates in low humidity environment, and faulty in charging may be caused at that part.
On the other hand, image forming apparatuses are known in which only DC voltage is applied on a charge roller (for example, see Japanese Patent Application Laid-open No. 2004-287027). However, in such an image forming apparatus, unevenness in charging is more likely to occur although deterioration in photoconductor and charge roller is smaller than the case using AC charging. This is attributable to the fact that charge potential of a photoconductor will converge to potential of DC current by repetition of positive charging and negative charging in the case of AC charging, whereas, when only DC voltage is applied, discharge occurs only in one direction according to the capacitor model. In other words, when DC voltage is applied to the charge roller while the photoconductor is not charged at all, large discharge current flows immediately after the application, and the discharge current fails to flow immediately as charge potential of the photoconductor increases. When surface resistance of photoconductor and resistance of the charge roller are perfectly uniform, and the assembly accuracy of the photoconductor and the charge roller is perfect, unevenness will not occur in charge potential of the photoconductor. However, when an image of such high quality exceeding 1000 dots per inch of writing to the photoconductor is to be formed, unevenness in charging of the photoconductor is inevitable, so that an image of high quality cannot be formed.
The present inventors made detailed observation to understand why charging unevenness occurs upon application of only DC voltage on a charge roller. A photoconductor and a contact-type charge roller are arranged, and the photoconductor is rotated with regard to one point (hereinafter, “point A”) on the photoconductor, and DC voltage is applied on the charge roller. According to the Paschen's law, discharge is started when the distance between point A and surface of the charge roller is equal to or less than a certain value. As the distance from surface of the charge roller reduces by rotation of point A, a threshold potential difference between the photoconductor and the charge roller where discharge occurs is small according to the Paschen's law, however, since potential at point A has been increased due to discharge theretofore, potential difference between point A and the charge roller is small, and discharge becomes difficult to occur. As point A further rotates and distance between the photoconductor and the charge roller is equal to or less than a certain value, discharge no longer occurs. Even when point A passes the position where the photoconductor and the charge roller contact each other, and further rotates to reach a distance that allows discharge according to the Paschen's law, potential at point A is already high and potential difference between point A and the charge roller is small, so that discharge rarely occurs. In this way, when only DC voltage is applied on the charge roller, discharge occurs only on the upstream side from the position where the photoconductor and the charge roller contact each other, and discharge occurs only continuously. When charging unevenness of the photoconductor widely spans on the downstream side from the position where the photoconductor and the charge roller contact each other, discharge may occurs, however, since the part of charging unevenness is typically in the form of small dots, discharge rarely occurs under the influence of a part where charging unevenness does not occur and charging potential is normal.
On the other hand, when DC voltage and AC voltage are overlapped on the charge roller, positive discharge and negative discharge occur a number of times in a second corresponding to frequency, and discharge occurs not only on the upstream side from the position where the photoconductor and the charge roller contact each other, but also on the downstream side, so that discharge occurs even in the part where discharge is difficult to occur, and charging potential is uniform.
The present inventors made further observation regarding potential change of the photoconductor when only DC voltage is applied on the charge roller, and found that discharge sometimes occurs on the downstream side from the position where the photoconductor and the charge roller contact each other when the linear velocity of the photoconductor is low. This would be attributed to the fact that when the photoconductor is charged by discharge, the charges do not remain in the part where discharge occurs, and a part of area that allows occurrence of discharge is created by disappearance of charges due to dark attenuation, or by dispersion of charges. We also found that for reconstruction of a part of area that allows discharge on the downstream side from the position where the photoconductor and the charge roller contact each other, a certain degree of time is required, and time for passage through the parts where discharge does not occur according to the Paschen's law before and after the position where the photoconductor and the charge roller contact each other is important.
According to the Paschen's law, discharge between conductors is indicated, however, since both the photoconductor and the charge roller include capacity components, potential difference that allows discharge is actually higher than that indicated by the Paschen's law, and distance between the photoconductor and the charge roller that allows discharge is actually wide, and differs depending on the image forming apparatus.
In view of the above, the present inventors made diligent efforts to locate the part where discharge can occur, and the part where discharge cannot occur, and found that when a photoconductor and a charge roller are disposed in contact with each other, and DC voltage is intermittently applied on the charge roller while the photoconductor and the charge roller are not rotated and the photoconductor is exposed to light, two lines (discharge mark) arise in the part where discharge occurs on the photoconductor surface.
The inventors found that when the center interval of discharge mark of the photoconductor (width of gap) and the linear velocity of the photoconductor fall within specified ranges, the charging potential of the photoconductor is uniform and a high quality image can be formed with high resolution, and accomplished the present invention.